This invention relates to a solar energy collecting device employing a low cost method for making highly efficient selective surface coatings, said device having the ability to reflect and/or concentrate solar energy.
Commercially available solar energy collectors of both tubular and the flat plate type, usually employ a blackened surface to facilitate the absorption of the maximum amount of solar radiation. Blackened surfaces, while being excellent absorbers of solar energy, usually have the drawback of being excellent emitters of radiation as well, and thus lowering the overall efficiency of the unit. Many high efficiency collectors overcome this problem by utilizing what is known in the industry as a solar selective surface coating on the absorber tube or plate. Most selective surfaces of a high quality nature; that is, having high absorption, low emission, no significant degradation with time, and little or no thermal degradation, are formed by blackening a chrome, nickel or as described in U.S. Pat. No. 4,339,484 to Harding, a sputtered carbide surface. Although effective, these coatings are expensive and contribute to a major portion of the overall cost of manufacturing the collector. The goal of any good collector is to attain the highest possible efficiency for the lowest possible cost.
An analysis of solar collectors reveals that the cost of material usually represents eighty to ninety-five percent of the direct manufacturing cost. It is inherently obvious that the easiest way of reducing the cost of manufacturing solar collectors is to reduce material cost. An expanded use of glass has been sought by many to keep down expenses, particularly by those involved in the research and manufacture of tubular or parabolic type collectors consisting of a tubular element or an array of such elements. Each element normally consists of a blackened absorber tube, usually metal, surrounded by a gas filled or an evacuated space with the entire assembly jacketed by a transparent wall, usually a glass tube of larger diameter. Because most elements use high vacuums with pressures of 10.sup.-4 Torr or lower, it is necessary to use materials with low outgassing characteristics.
Stainless steel absorber tubes are typically used. They have the advantage of having the selective surface easily applied by electroplating and have low outgassing rates, but have the disadvantage of being very expensive. The cost of stainless steel tubing is roughly two to four times greater than glass. A significant drawback of using non-glass absorber tubes concerns, because of the wide temperature excursions in the collector and the wide differences in thermal expansion coefficients between the outer glass sheet and the inner metal absorption tube, providing means to accommodate the differences in expansion.
A system disclosed by U.S. Pat. No. 1,855,815 to Abbot addressed this problem back in 1934 with the use of expandable bellows. Many collectors today are manufactured using bellows and/or keepers, however, they are relatively expensive and require the use of glass-to-metal seals which are both difficult to apply and costly.
U.S. Pat. No. 4,151,828 to Mather et al addresses this same problem by sealing with O-rings instead of a direct weld or seal. This solved the expansion problem, however, a person experienced in vacuum technology is aware that O-rings will not hold a hard vacuum of the nature of 10.sup.-4 Torr or less for long durations, not even with the use of getters. A vacuum with pressures higher than 10.sup.-4 Torr may be held for a short time period relative to the expected life of the collector, but vacuum of pressures greater than 10.sup.-4 Torr have proved to be totally ineffective in reducing heat losses either by conduction and/or convection, and therefore, provide no significant insulating capabilities in the solar collector.
The problem of differential expansions between the absorber and the outer glass jacket have been solved by others by using glass as the material of the absorber tube. The glass absorber tube can be easily welded to the outer glass jacket and provides the best possible vacuum seal with the lowest outgassing rate. The glass tubing is also significantly less costly than stainless steel tubing. Other metals such as brass, copper, steel, or aluminum are either too porous or outgas too much to be practical.
Although using a glass absorber has solved many problems inherent in tubular elements, it has created another problem. A selective surface can be easily applied to metal, but such is not the case with glass. For metal absorbers, usually a coating of one or two metals is electroplated onto the tube with the outer metal coating, usually nickel or chrome, chemically blackened in a way to provide optimum optical characteristics. The problem with glass tubing is that it is very difficult to get a metal coating on glass, especially one that has good adhesion and good thermal contact.
Traditionally, skilled artisans have used either evaporative or sputtering techniques. The aforementioned Harding technique uses sputtered carbides. U.S. Pat. No. 4,016,860 to Moan uses vapor deposition. These two mentioned techniques, although effective, require the use of expensive vacuum equipment and raw materials, and hence, the coating is too expensive to be practical.
The Department of Energy, Sandia Laboratories, New Mexico, has experimented with attaching foils with a selective surface to the glass. Although adhesives cannot be used because of high outgassing rates, it tried two other methods, one being glass frits and the other using electrostatic bonding techniques. In both cases, the foils did not provide good thermal contact with the glass and the processes proved too expensive to be practical. Other companies have used spray-on coatings, but optical characteristics and outgassing rates are poor.
U.S. Pat. No. 4,375,807 to Friederich et al describes an elaborate process which is commonly known by those in the electronics industry as organo-metallics. This process, although somewhat effective, is also too expensive to be practical.
Many tubular collectors now being produced have an arrangement to concentrate solar radiation onto the absorber tube by reflection from a surface under or adjacent to the absorber tube. The reflective surface is usually applied to either the inner or outer surface of a transparent glass jacket on the underside or by an entirely separate reflective material arranged in a parabolic shape under the absorber unit with the absorber located at the focal point of the reflective surface. In the first case, reflective metals are vapor deposited or chemically deposited on the glass jacket which, as in the case of the selective surface, has proved to be too expensive to be practical. In the latter case, reflective material forming a parabolic shape are employed by using highly reflective materials which proved once again to be too costly to be practical. An example of such a material is electropolished aluminum with an anodized or acylic coated surface. Another material that is widely used is aluminized polyester which has a relatively low reflection coefficient, and hence results in an overall low efficiency of the unit, and increases the overall cost of the system.
An object of this invention is to provide a means for significantly lowering the overall cost of solar collectors by providing low cost methods of applying a selective surface to most materials, glass in particular, and yet retain a high absorption efficiency of 70% or better with less than 5% infrared emission rates. At the same time, this selective surface must have good thermal contact with its substrate, little or no thermal or ultraviolet degradation, and low outgassing properties.
It is a further goal of this invention to provide a low cost, highly reflective surface than can be applied to glass or other materials.
Another goal of this invention is to provide an improved method for circulation of the thermal absorption fluid in units that use phase change processes, such as Freon, to transfer heat.
A yet further goal of this invention is to enhance the performance of a getter by a factor of ten to one hundred, by increasing the longevity of the collector, and diminishing the size and cost of the getter.